1. Field of the Invention
The present invention relates to a thermal recording apparatus and, more particularly, to a pulse generation circuit of a thermal recording apparatus that allows facsimile machines, printers and other related devices to provide recordings thermally on sheets of thermosensitive paper.
2. Description of Related Art.
A thermal recording apparatus (also hereinafter referred to as a thermal printer), uses heat to make recordings of information on paper. Thermal characteristics such as the ambient temperature and the thermal head temperature significantly affect recording quality, particularly the thickness of printing. Thus, to obtain stable and high levels of recording requires temperature compensation to be carried out. To provide the compensation requires detecting the head temperature of the thermal printer by a temperature sensor. The temperature sensor usually includes an element such as a thermistor which has a specific temperature-resistance characteristic (this element is hereinafter also referred to as a heat sensitive resistor).
A typical circuit configuration for temperature compensation comprises a resistance (R) and a capacitor (C), constituting an RC charging-discharging circuit. A heat sensitive resistor, which may be a thermistor, is connected to this circuit to vary the charging-discharging waveform of the circuit. The thermistor is disposed on the thermal head. As the head temperature varies, so does the degree of resistance of the thermistor. Because thermistors become less resistant at higher temperatures and have higher levels of resistance at lower temperatures, the time constant for the RC charging-discharging waveform is smaller at higher temperatures and larger at lower temperatures. That is, the charging time required until the output voltage of the RC charging-discharging circuit reaches its threshold is longer when the thermal head temperature is lower and shorter when the head temperature is higher. Since the pulse duration of the output signal coming from the temperature compensation circuit is proportional to the charging time, the output signal coming from the temperature compensation circuit has pulses of longer duration when the thermal head bears lower temperatures, and has pulses of shorter duration when the head bears higher temperatures.
Given these constraints, when it is desired to vary the recording energy due to a change in recording speed, prior art temperature compensation circuits have been typically capable of outputting only one type of recording pulse at a certain temperature.